Government legislators blamed their "fascist" rivals for starting the violence, which illustrated the volatile state of politics in the OPEC nation after the death of late socialist leader Hugo Chavez last month.

"We knew the opposition came to provoke violence," Maduro said of the incident. "This must not be repeated."

The 50-year-old Maduro, who was Chavez's chosen successor, defeated opposition candidate Henrique Capriles by 1.5 percentage points. Capriles, 40, has refused to recognize his victory, alleging that thousands of irregularities occurred and the vote "stolen."

The vote exposed a nation evenly divided after 14 years of Chavez's hardline socialist rule.

"They can beat us, jail us, kill us, but we will not sell out our principles," one of the opposition parliamentarians, Julio Borges, told a local TV station, showing a bruised and bloodied face. "These blows give us more strength."

One assembly worker, who asked not to be named, told Reuters the trouble began when opposition legislators shouted "fascist" at the National Assembly leader and unfolded a protest banner reading "parliamentary coup."

Government parliamentarians attacked them. Laptops and tables were hurled in the ensuing melee, with one legislator hit over the head with a chair, the witness said.

Workers later had to show their phones to see if they had photos or videos of the incident, the assembly employee added.

'Defending Chavez's Legacy'

Government parliamentarian Odalis Monzon said she and some colleagues were attacked and beaten. "Today again I had to defend the commander's (Chavez's) legacy," she said.

The fracas came after the government-controlled assembly passed a measure denying opposition members the right to speak in the chamber until they recognized Maduro as president.

"Until they recognize the authorities, the institutions of the republic, the sovereign will of our people, the opposition deputies will have to go and speak (to the private media) but not here in this National Assembly," said Diosdado Cabello, the head of parliament.

Both sides accused each other of starting the incident, which took place behind closed doors without media present.

In a video that pro-opposition private TV station Globovision broadcaster said it obtained from a parliamentarian, various assembly members could be seen hitting each other and scuffling to cries of "stop" from others.

In another potential flashpoint for Venezuela, the government and opposition are planning rival marches in Caracas on Wednesday to commemorate May Day.

Former colonial ruler Spain this week offered to mediate in Venezuela's political tensions. But Maduro rejected that.

"Stop sticking your noses in Venezuela. Spanish foreign minister, get out, you impertinent man. Venezuela is to be respected," he said in a speech, referring to Foreign Minister Jose Manuel Garcia-Margallo.